The use of large format display systems is widely spread and finds applications in control rooms, for advertising, for displaying at events, etc. Most large format display systems used are tiled display systems, constructed out of a number of display tiles or display modules. The latter allows to obtain larger display surfaces than obtainable using conventional displays, by placing a plurality of display tiles adjacent to each other and spread the lighting or imaging output over the different display tiles. The display tiles or modules are mechanically coupled together in order to form the full large format display. The space or gap between the different tiles or modules of the display is called a seam.
One of the main specifications of display quality is uniformity in color and brightness. For a tiled display, obtaining color and brightness uniformity is often even more difficult, because the tiles and their seams form a regular structure, which is very easily detected by the human eye. Current displays may have a large bit-depth (16 bits and more) so that they can be very smooth after color/brightness calibration. As a result the tiled display looks very smooth overall. Nevertheless, it is known that if a human eye observes a uniform plane, even the smallest local non-uniformities, such as a small variance on the mechanical seams become visible.
Visibility of seams can occur for a number of reasons. One cause of visibility of the seams is the change in pitch between pixels of neighbouring tiles compared to the pitch between pixels of one tile, which is amongst others determined by the alignment of the display tiles. The seam appears as a dark line if the gap between display tiles increases the pitch between individual light sources (e.g. LED's) or pixels at the edge of neighbouring tiles compared to the pitch between individual light sources or pixels within one tile. The seam appears as a bright line at the crossing between display tiles, if the gap between display tiles decreases the pitch between individual light sources (e.g. LED's) at the edge of neighbouring tiles compared to the pitch between individual light sources or pixels within one tile. In the first case the energetic light flux at the mechanical connection of the display tiles decreases, in the second case the energetic light flux increases. Therefore it is advantageous to keep the pitch between individual light sources constant even over the edges of neighbouring tiles. Another cause of visibility of seams between tiles may be the presence of environmental or stray light, e.g. incident on the backside of the tiled display and guided through the seams in the viewing direction. Mechanical means for hiding the seam are known.
A number of correction techniques are known for correction of visual artifacts in digital images. One example thereof is the use of error diffusion, whereby a error for one pixel is diffused by spreading an error signal or part thereof to neighbouring pixels or to the same pixel in a following frame. Another known method, e.g. as known from U.S. Pat. No. 6,040,876, is the provision of dithering for compensation of image contouring effects caused by quantization errors in digital imaging. The dithering may be especially suitable for reducing false color contouring, by applying the dithering to the chrominance portion of a digital color image signal. The dithering is applied by adding a small amount of noise or pseudo-noise to the image.